LONDON — A standoff between the British government and The Guardian over national security and press freedom entered its latest round Friday when the high court extended police powers to analyze encrypted material seized from the partner of one of the newspaper’s journalists this month.

The decision, which slightly expanded the authority of the police to investigate the digital files, came after a senior national security adviser to Prime Minister David Cameron told the court that a computer hard drive and memory sticks confiscated from David Miranda, the partner of Glenn Greenwald, a columnist for The Guardian, may contain the identities of intelligence officers.

The adviser, Oliver Robbins, said in a written statement that the information could put the officers and their families at risk, or even make them vulnerable to recruitment by foreign intelligence services.

The 13-page statement was the first detailed explanation the government had provided for its detention of Miranda at Heathrow Airport on Aug. 18. He was questioned for nearly nine hours under terrorism legislation, and encrypted digital files he was carrying were seized.

Greenwald has led the reporting on national security material leaked by Edward J. Snowden, the former contractor for the U.S. National Security Agency. Miranda, 28, was detained as he was bound for Brazil, where he and Greenwald live. He had met in Berlin with a U.S. filmmaker who has been working with Greenwald on the NSA coverage.

Robbins, the deputy national security adviser on intelligence, security and resilience, said in the statement provided to the court that the government had so far viewed only a portion of the encrypted files seized from Miranda. In all, he said, the files contained about 58,000 highly classified documents, which were “highly likely to describe techniques which have been crucial in life-saving counterterrorist operations, and other intelligence activities vital to U.K. national security.” Allowing the material to become public, he said, “would do serious damage to U.K. national security and ultimately risk lives.”

In a statement, Morgan accused the government and the police of making “sweeping assertions about national security threats which they said entitled them to look at the materials seized, but they have said that they cannot provide further details in open court.”